Wireless chemical sensor and sensing method for use therewith

ABSTRACT

A wireless chemical sensor includes an electrical conductor and a material separated therefrom by an electric insulator. The electrical conductor is an unconnected open-circuit shaped for storage of an electric field and a magnetic field. In the presence of a time-varying magnetic field, the first electrical conductor resonates to generate harmonic electric and magnetic field responses. The material is positioned at a location lying within at least one of the electric and magnetic field responses so-generated. The material changes in electrical conductivity in the presence of a chemical-of-interest.

ORIGIN OF THE INVENTION

The invention was made in part by an employee of the United StatesGovernment and may be manufactured and used by or for the Government ofthe United States of America for governmental purposes without thepayment of any royalties thereon or therefor. Pursuant to 35 U.S.C.§119, the benefit of priority from provisional application 61/051,841,with a filing date of May 9, 2008, is claimed for this non-provisionalapplication.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is co-pending with the related patentapplication entitled “WIRELESS CHEMICAL SENSOR AND SENSING METHOD FORUSE THEREWITH”, filed on the same day and owned by the same assignee asthis patent application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to chemical sensors. More specifically, theinvention is a wireless chemical sensor that includes a material whoseelectrical conductivity changes in the presence of a chemical ofinterest, where such electrical conductivity changes a harmonic responseof a spaced-apart electrically-unconnected geometric pattern that iselectrically conductive.

2. Description of the Related Art

Chemical sensors have been employed for a large variety of applicationssuch as bio-sensing, environmental analysis, food analysis, clinicaldiagnostics, drug detection, gas detection, toxicity detection, anddetection of chemicals that could be used for warfare or terrorism. Inone approach, sensors have a specific synthesized receptor thatselectively binds with an analyte of interest. Another sensor approachis to have a specific chemical reactant react with a target reactant.Each approach produces a measurable change that is discernable via anelectrical component such as a capacitor or resistor. Typically, thereceptor/reactant must physically contact some part(s) of the electricalcomponent(s). This can limit the number of applications that couldutilize chemical sensors.

Chemical sensor innovation is driven by either the infrastructureinnovations such as microelectromechanical or wireless sensors, orinnovations/discoveries in chemistry such as the development ofCarbon-60 that resulted in carbon nanotubes and the development ofconductive polymers. Newer sensor baseline circuit designs includemagnetic field response sensors that require no physical connections toa power source or acquisition hardware. For example, U.S. Pat. Nos.7,086,593 and 7,159,774 disclose magnetic field response sensorsdesigned as passive inductor-capacitor circuits and passiveinductor-capacitor-resistor circuits that produce magnetic fieldresponses whose harmonic frequencies correspond to states of physicalproperties of interest. A closed-circuit magnetic field response sensoris made by electrically connecting a spiral trace inductor to aninterdigitated electrode capacitor or capacitor plates. A magnetic fieldresponse recorder wirelessly transmits a time-varying magnetic fieldthat powers each sensor using Faraday induction. Each sensor thenelectrically oscillates at a resonant frequency that is dependent uponthe capacitance, inductance and resistance of each sensor. Thefrequency, amplitude and bandwidth of this oscillation are wirelesslysensed by the magnetic field response recorder. The sensor's response isindicative of a parameter that is to be measured.

While the above-described magnetic field response measurementacquisition system greatly improves the state-of-the-art of wirelesssensing, electrical connections are still required between the sensor'sinductor and capacitor. Such connections are subject to breakage,especially if the sensor will undergo flexing during its useful life.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide awireless chemical sensor.

Another object of the present invention is to provide a wirelesschemical sensor that need not expose any electrical components thereofto a chemical environment being monitored.

Still another object of the present invention is to provide a wirelesschemical sensor that minimizes required components in order to minimizefailures as well as cost.

Yet another object of the present invention is to provide a wirelesschemical sensor that is functional after many types of damaging events.

Other objects and advantages of the present invention will become moreobvious hereinafter in the specification and drawings.

In accordance with the present invention, a wireless chemical sensorincludes an electrical conductor, a material spaced apart from theelectrical conductor, and an electric insulator disposed between theelectrical conductor and the material. The electrical conductor hasfirst and second ends and is shaped between the first and second endsfor storage of an electric field and a magnetic field. The first andsecond ends remain electrically unconnected such that the electricalconductor so-shaped defines an unconnected open-circuit havinginductance and capacitance. In the presence of a time-varying magneticfield, the electrical conductor so-shaped resonates to generate harmonicelectric and magnetic field responses, each of which has a frequency,amplitude and bandwidth associated therewith. The material is spacedapart from the electrical conductor at a location lying within at leastone of the electric and magnetic field responses so-generated. Thematerial changes in electrical conductivity in the presence of achemical-of-interest. The change in conductivity results in a change tothe conductor's generated harmonic electric and magnetic fieldresponses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a wireless chemical sensor in accordancewith an embodiment of the present invention;

FIG. 2 is a schematic view of a magnetic field response recorder used inan embodiment of the present invention;

FIG. 3 is a schematic view of a wireless chemical sensor to include afield response recorder in accordance with another embodiment of thepresent invention;

FIG. 4 is a schematic view of a spiral trace conductor pattern whosetraces are non-uniform in width;

FIG. 5 is a schematic view of a spiral trace conductor pattern havingnon-uniform spacing between the traces thereof;

FIG. 6 is a schematic view of a spiral trace conductor pattern havingnon-uniform trace width and non-uniform trace spacing; and

FIG. 7 is a schematic view of a wireless chemical sensor in accordancewith another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and more particularly to FIG. 1, awireless chemical sensor in accordance with an embodiment of the presentinvention is shown and is referenced generally by numeral 100. Sensor100 is constructed to be sensitive to the presence of achemical-of-interest that can be in the form of a solid, liquid or gaswithout departing from the scope of the present invention. In theillustrated embodiment, sensor 100 includes an unconnected electricalpattern 10, an electrical insulator 20, and a material 30 whoseelectrical conductivity is altered (e.g., increased, decreased, reducedto near zero, etc.) when in the presence of a chemical-of-interest (notshown). Accordingly, material 30 is selected to be a material that isinitially conductive, partially conductive, or non-conductive, but whoseelectrical conductivity changes when in the presence of achemical-of-interest.

As will be explained further below, it is the change in electricalconductivity of material 30 that allows sensor 100 to be sensitive tothe chemical-of-interest. Material 30 can be in the form of a sheetspanning the area of pattern 10 and adapted to be mounted where it isneeded. However, the present invention is not so limited as material 30could also be in the form of a thin strip overlaying some region ofpattern 10. The change in electrical conductivity of material 30 in thepresence of the chemical-of-interest can be caused by a chemicalreaction between material 30 and the chemical-of-interest, or absorptionof the chemical-of-interest by material 30.

Electric insulator 20 is any material/structure that electricallyinsulates pattern 10 from material 30 in all operating conditions toinclude those conditions when the chemical-of-interest is present.Electric insulator 20 can be a structural element/substrate on whichpattern 10 and material 30 are mounted such that sensor 100 is apre-fabricated sensor with each element thereof being flexible orinflexible to suit a particular application. However, electric insulator20 can also be a structure inherent in an environment where pattern 10and material 30 will be installed. For example, if achemical-of-interest was to be monitored in the air outside of abuilding, electric insulator 20 could be a window in the building.Material 30 would then be mounted on the outside of the window andpattern 10 would be mounted on the inside of the window oppositematerial 30. Still further, electric insulator 20 could simply be an airgap disposed between pattern 10 and material 30.

Electrical conductor pattern 10 is any electrical conductor (e.g., wire,run, thin-film trace, etc.) that can be shaped to form an open-circuitpattern that can store an electric field and a magnetic field. The term“open-circuit pattern” as used herein means that the conductor has twoends that are electrically unconnected so that the resulting conductorpattern is an electrical open circuit having inductance and capacitanceattributes.

Pattern 10 can be a stand-alone electrically-conductive run. Pattern 10can also be made from an electrically-conductive run or thin-film tracethat can be deposited directly onto insulator 20 or on an optionalsubstrate material 22 (referenced by dashed lines to indicate theoptional nature thereof) that is electrically insulating andnon-conductive. The particular choice of the substrate material willvary depending on how it is to be attached to insulator 20 or otherwisemounted in its desired location. Although not a requirement of thepresent invention, the surface on which pattern 10 is deposited istypically a planar surface. Techniques used to deposit pattern 10 eitherdirectly onto insulator 20 or on a substrate material can be anyconventional metal-conductor deposition process to include thin-filmfabrication techniques. As will be explained further below, pattern 10can be constructed to have a uniform or non-uniform width, and/oruniform or non-uniform spacing between adjacent portions of thepattern's runs/traces.

The basic features of pattern 10 and the principles of operation forsensor 100 will be explained for a spiral-shaped conductor pattern.However, it is to be understood that the present invention could bepracticed using other geometrically-patterned conductors provided thepattern has the attributes described herein. The basic features of aspiral-shaped conductor that can function as pattern 10 are described indetail in U.S. Patent Publication No. 2007/0181683, the contents ofwhich are hereby incorporated by reference in their entirety. Forpurpose of a complete description of the present invention, the relevantportions of this publication will be repeated herein.

As is well known and accepted in the art, a spiral inductor is ideallyconstructed/configured to minimize parasitic capacitance so as not toinfluence other electrical components that will be electrically coupledthereto. This is typically achieved by increasing the spacing betweenadjacent conductive portions or runs of the conductive spiral pattern.However, in the present invention, pattern 10 is constructed/configuredto have a relatively large parasitic capacitance. The capacitance ofpattern 10 is operatively coupled with the pattern's inductance suchthat magnetic and electrical energy can be stored and exchanged by thepattern. Since other geometric patterns of a conductor could alsoprovide such a magnetic/electrical energy storage and exchange, it is tobe understood that the present invention could be realized using anysuch geometrically-patterned conductor and is not limited to aspiral-shaped pattern.

The amount of inductance along any portion of a conductive run ofpattern 10 is directly related to the length thereof and inverselyrelated to the width thereof. The amount of capacitance between portionsof adjacent conductive runs of pattern 10 is directly related to thelength by which the runs overlap each other and is inversely related tothe spacing between the adjacent conductive runs. The amount ofresistance along any portion of a conductive run of pattern 10 isdirectly related to the length and inversely related to the width of theportion. Total capacitance, total inductance and total resistance for aspiral pattern are determined simply by adding these values from theindividual portions of the pattern. The geometries of the variousportions of the conductive runs of the pattern can be used to define thepattern's resonant frequency.

Pattern 10 with its inductance operatively coupled to its capacitancedefines a magnetic field response sensor. In the presence of atime-varying magnetic field, pattern 10 electrically oscillates at aresonant frequency that is dependent upon the capacitance and inductanceof pattern 10. This oscillation occurs as the energy is harmonicallytransferred between the inductive portion of pattern 10 (as magneticenergy) and the capacitive portion of pattern 10 (as electrical energy).That is, when excited by a time-varying magnetic field, pattern 10resonates a harmonic electric field and a harmonic magnetic field witheach field being defined by a frequency, amplitude, and bandwidth.

The application of a magnetic field to pattern 10, as well as thereading of the induced harmonic response at a resonant frequency, can beaccomplished by a magnetic field response recorder. The operatingprinciples and construction details of such a recorder are provided inU.S. Pat. Nos. 7,086,593 and 7,159,774, the contents of which are herebyincorporated by reference in their entirety. Briefly, as shown in FIG.2, a magnetic field response recorder 50 includes a processor 52 and abroadband radio frequency (RF) antenna 54 capable of transmitting andreceiving RF energy. Processor 52 includes algorithms embodied insoftware for controlling antenna 54 and for analyzing the RF signalsreceived from the magnetic field response sensor defined by pattern 10.On the transmission side, processor 52 modulates an input signal that isthen supplied to antenna 54 so that antenna 54 produces either abroadband time-varying magnetic field or a single harmonic field. On thereception side, antenna 54 receives harmonic magnetic responses producedby pattern 10. Antenna 54 can be realized by two separate antennas or asingle antenna that is switched between transmission and reception.

In operation, when pattern 10 is exposed to a time-varying magneticfield (e.g., as generated by recorder 50), pattern 10 resonates harmonicelectric and magnetic fields. The generated magnetic field is generallyspatially larger than the generated electric field. Material 30 ispositioned relative to pattern 10 such that it will lie within one orboth of the generated magnetic and electric fields. By way of example,the operation of sensor 100 will be described relative to the generatedmagnetic field emanating from pattern 10 when it is exposed to atime-varying magnetic field.

For fixed excitation conditions, the magnetic field response frequency,amplitude, and bandwidth of pattern 10 are dependent upon the electricconductivity of any material placed within its magnetic field. That is,when a material having electrical conductivity properties (e.g.,material 30) is placed inside either the generated magnetic field orelectric field of pattern 10, the generated fields around pattern 10 areattenuated more when the conductivity of material 30 increases (afterbeing exposed to the chemical-of-interest) and attenuated less when theconductivity of material 30 decreases (after being exposed to thechemical-of-interest). The energy lost from the generated magnetic fieldand electric field will alter the magnetic field response frequency,amplitude and bandwidth of pattern 10. More specifically, since there isless energy in the generated magnetic field, pattern 10 exhibits lowerinductance and capacitance, and produces a lower response amplitude.Accordingly, if the relative positions of pattern 10 and material 30remain fixed and if the conductivity of material 30 is fixed, then themagnetic field response of sensor 100 remains unchanged for fixedexcitation conditions. These fixed conditions and resulting magneticfield response of sensor 100 define a baseline response for sensor 100that is recorded prior to using sensor 100.

In accordance with the present invention, material 30 is a conductivematerial that will experience a change in electrical conductivity in thepresence of a chemical-of-interest (e.g., via chemical reaction,chemical absorption, etc.). Accordingly, the above-described baselineresponse of sensor 100 is recorded in conditions where thechemical-of-interest is not present. Then, when material 30 issubsequently exposed to a chemical-of-interest, its electricalconductivity is altered to thereby change the magnetic field response ofsensor 100 in a corresponding fashion. For example, in the case of asheet of material 30 overlaying pattern 10, if the presence of thechemical-of-interest causes material 30 to become less conductive,pattern 10 will lose less energy resulting in a decreased response inbandwidth, decreased frequency, and increased response in amplitude.Thus, the magnetic field response of sensor 100 can be used to detectthe presence of the chemical-of-interest. Once the baseline response ofsensor 100 is known and sensor 100 is placed in use,interrogation/monitoring of sensor 100 (for changes in response relativeto the baseline response) can be carried out continuously, periodically,on-demand, etc., without departing from the scope of the presentinvention.

As mentioned above, a magnetic field response recorder can be used tosupply the time-varying magnetic field used to excite pattern 10 and toread/record the generated magnetic field provided by pattern 10.However, the present invention is not so limited since the excitationtime-varying magnetic field also causes an electric field to be producedby pattern 10. If material 30 were positioned to lie within the electricand magnetic field responses of pattern 10 (e.g., through proper sizingof insulator 20), one or both of the field responses could be monitored.Accordingly, FIG. 3 illustrates another embodiment of the presentinvention where pattern 10 of sensor 100 is excited and monitored by afield response recorder 60. Recorder 60 transmits the excitationmagnetic field to pattern 10 and monitors one or both of the generatedmagnetic and electric field responses of pattern 10 if material 30 islocated close enough to pattern 10 to lie in both the magnetic andelectric field responses. In terms of the electric field response,recorder 60 monitors the frequency, amplitude and bandwidth of theelectric field response.

Also as mentioned above, both the width of the pattern's conductiveruns/traces and the spacing between adjacent portions of the conductiveruns/traces can be uniform. However, the present invention is not solimited. For example, FIG. 4 illustrates a spiral pattern 40 in whichthe width of the conductive trace is non-uniform while the spacingbetween adjacent portions of the conductive trace is uniform. FIG. 5illustrates a spiral pattern 42 in which the width of the conductivetrace is uniform, but the spacing between adjacent portions of theconductive trace is non-uniform. Finally, FIG. 6 illustrates a spiralpattern 44 having both a non-uniform width conductive trace andnon-uniform spacing between adjacent portions of the conductive trace.

The wireless chemical sensor of the present invention can be configuredin other ways than described above without departing from the scope ofthe present invention. For example, FIG. 7 illustrates a sensor 102 inwhich its unconnected electrical conductor pattern 10 is encased inelectric insulator 20. In this embodiment, insulator 20 also protectspattern 10 from environmental conditions. Material 30 can be positionedadjacent to insulator 20 or can be coupled thereto. The operation ofsensor 102 is the same as described above.

The present invention is further discussed in Woodard, Olgesby, Taylorand Shams, “Chemical Detection using Electrically Open Circuits havingno electrical Connections,” IEEE Sensors 2008, 26-29 Oct. 2008, herebyincorporated by reference in its entirety.

The advantages of the present invention are numerous. The wirelesschemical sensor requires only a simple unconnected, open-circuitconductor shaped to store electric and magnetic fields, and a materialthat experiences a change in conductivity in the presence of achemical-of-interest. The material is simply spaced apart from theshaped conductor by air, some non-conductive structure inherent in theapplication environment, or by an insulator/substrate that also servesas the means to “package” the shaped conductor to thereby form aprefabricated wireless chemical sensor. The wireless chemical sensorrequires no electrically connected components, is simple to produce, andcan be excited/powered using known field response recorder technology.The shaped conductor and material can be separated such that only thematerial need be exposed to a potentially harsh chemical environment.

Although the invention has been described relative to a specificembodiment thereof, there are numerous variations and modifications thatwill be readily apparent to those skilled in the art in light of theabove teachings. It is therefore to be understood that, within the scopeof the appended claims, the invention may be practiced other than asspecifically described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A wireless chemical sensor, comprising: anelectrical conductor having first and second ends and shaped to form apattern between said first and second ends for storage of an electricfield and a magnetic field, said first and second ends remainingelectrically unconnected such that said electrical conductor so-shapeddefines an unconnected open-circuit having inductance and capacitancewherein, in the presence of a time-varying magnetic field, saidelectrical conductor so-shaped resonates to generate harmonic electricand magnetic field responses, each of which has a frequency, amplitudeand bandwidth associated therewith; a material entirely spanning saidpattern while being spaced apart from said electrical conductor at alocation lying within at least one of said electric and magnetic fieldresponses so-generated, said material selected to affect attenuation ofenergy associated with said electric and magnetic field responsesso-generated in the presence of a chemical-of-interest; and an electricinsulator disposed between said electrical conductor and said material.2. A wireless chemical sensor as in claim 1, further comprising amagnetic field response recorder for wirelessly transmitting saidtime-varying magnetic field to said electrical conductor and forwirelessly detecting said frequency, amplitude and bandwidth associatedwith said magnetic field response so-generated.
 3. A wireless chemicalsensor as in claim 1, further comprising an electric field responserecorder for wirelessly transmitting said time-varying magnetic field tosaid electrical conductor and for wirelessly detecting said frequency,amplitude and bandwidth associated with said electric field responseso-generated.
 4. A wireless chemical sensor as in claim 1, wherein saidelectrical conductor comprises a thin-film trace.
 5. A wireless chemicalsensor as in claim 4, wherein said trace is uniform in width.
 6. Awireless chemical sensor as in claim 4, wherein spacing between adjacentportions of said trace is uniform.
 7. A wireless chemical sensor as inclaim 4, wherein said trace is non-uniform in width.
 8. A wirelesschemical sensor as in claim 4, wherein spacing between adjacent portionsof said trace is non-uniform.
 9. A wireless chemical sensor as in claim1, wherein said material is adapted to chemically react with thechemical-of-interest.
 10. A wireless chemical sensor as in claim 1,wherein said material is adapted to absorb the chemical-of-interest. 11.A wireless chemical sensor as in claim 1, wherein said materialcomprises a sheet thereof.
 12. A wireless chemical sensor as in claim 1,wherein said electric insulator comprises air.
 13. A wireless chemicalsensor as in claim 1, wherein said electric insulator encases saidelectrical conductor.
 14. A wireless chemical sensor as in claim 1,wherein said electrical conductor and said material are coupled to saidelectric insulator.
 15. A wireless chemical sensor as in claim 1,wherein said electrical conductor, said material, and said electricinsulator are flexible.
 16. A wireless chemical sensor, comprising: anelectrical conductor in the form of a thin-film trace having first andsecond ends and shaped to form a pattern that can store an electricfield and a magnetic field, said first and second ends remainingelectrically unconnected such that said electrical conductor so-shapeddefines an unconnected open-circuit having inductance and capacitancewherein, in the presence of a time-varying magnetic field, saidelectrical conductor so-shaped resonates to generate harmonic electricand magnetic field responses, each of which has a frequency, amplitudeand bandwidth associated therewith; a material entirely spanning saidpattern while being spaced apart from said electrical conductor at alocation lying within at least one of said electric and magnetic fieldresponses so-generated, said material selected to affect attenuation ofenergy associated with said electric and magnetic field responsesso-generated in the presence of a chemical-of-interest; an electricinsulator disposed between said electrical conductor and said material;and a field response recorder for wirelessly transmitting saidtime-varying magnetic field to said electrical conductor and forwirelessly detecting said frequency, amplitude and bandwidth associatedwith at least one of said electric and magnetic field responsesso-generated.
 17. A wireless chemical sensor as in claim 16, whereinsaid trace is uniform in width.
 18. A wireless chemical sensor as inclaim 16, wherein spacing between adjacent portions of said trace isuniform.
 19. A wireless chemical sensor as in claim 16, wherein saidtrace is non-uniform in width.
 20. A wireless chemical sensor as inclaim 16, wherein spacing between adjacent portions of said trace isnon-uniform.
 21. A wireless chemical sensor as in claim 16, wherein saidmaterial is adapted to chemically react with the chemical-of-interest.22. A wireless chemical sensor as in claim 16, wherein said material isadapted to absorb the chemical-of-interest.
 23. A wireless chemicalsensor as in claim 16, wherein said material comprises a sheet thereof.24. A wireless chemical sensor as in claim 16, wherein said electricinsulator comprises air.
 25. A wireless chemical sensor as in claim 16,wherein said electric insulator encases said electrical conductor.
 26. Awireless chemical sensor as in claim 16, wherein said electricalconductor and said material are coupled to said electric insulator. 27.A wireless chemical sensor as in claim 16, wherein said electricalconductor, said material, and said electric insulator are flexible.